It seems to be some issue with custom JS on the main page in general; not just the countdown. If you look on the right rail, DiscordIntegrator also isn't working. However, they do work in the preview. I would suggest contacting staff.
The countdown is impacted because of how JS is loaded. Custom JS is loaded using the ResourceLoader (a.k.a. load.php) which combines the scripts into a single script. When your browser executes a script, it will halt execution if it encounters an error. Because the DiscordIntegrator part of the combined script is throwing an error, your browser stops executing the script before it gets to the Countdown portion. Thus, no countdowns.
It is for a similar reason that users can experience loading issues with VisualEditor because of mistakes in their personal JS.